A few years ago, I coined the expression “Cosford’s Law.” There was a gap in understanding from neophyte hydraulic specialists that hydraulic actuators moved because flow happened, rather than because force was applied. The truth, of course, is that no mechanical motion occurs without some force being applied.
Cosford’s Law states that “pressure makes it go, and flow is the rate in which you can create force.” I created this not because of narcissism, but to bridge the gap in understanding created by the misuse of the false expression, “flow makes it go.”
I’ve come to another gap in understanding, but this time related to cavitation. Cavitation is even more misunderstood than is the pressure vs flow juxtaposition. I need to be honest with you; cavitation is absolutely, perfectly harmless. Cavitation, that is, little bubbles created from vacuum, are entirely harmless in their genesis. Some understanding of cavitation helps prove my claim that cavitation is innocent.
A cavitation bubble pops into existence spontaneously when its liquid home can no longer keep the gas in saturation. All liquid has dissolved gases (typically just air), and the amount of dissolved gas depends partially on the pressure applied to the liquid. The vast majority of the time, liquids are exposed only to atmospheric pressure. But as pressure increases, more gas can be held in saturation, and as pressure decreases, less gas can be held in saturation. The absolute pressure is less important than the change, because as pressure drops, and the gas releases, it does so in the form of bubbles in the liquid.
Cavitation is the creation of air bubbles; that’s it. Cavitation is no more harmful than the bubbles blown by your 4-year-old daughter. If you took the outlet hose of a cavitating pump and let it flow freely into a bucket, absolutely no harm is done to the pump. It’s when the bubbles reach the discharge side of the pump under high pressure that problems occur as a result of cavitation.
When a gas bubble reaches the pressure side of the pump, it implodes as a super heated jet, and if the bubble was on the surface of metal, it damages that surface. It doesn’t matter if the bubbles were created spontaneously through cavitation, or from aeration, which is when air is drawn into to the inlet plumbing due to vacuum; either condition can cause the bubbles that do damage.
My problem is this; what are the little super-heated implosion jets called? It is this effect, not the spontaneously created bubbles, that cause damage. When you hear that tell-tale sound of ball-bearings rattling loose in your pump, our first response shouldn’t be “it’s cavitating,” but rather, “it’s super-heated implosion jetting.” This description is more accurate, but lacks the quality of a radio ad jingle.
Does anyone know the name of this effect? If not, I may have to conjure up a name for it, just as I have for two of my other creations. One of these is Cosford’s Law, of course. And the other? The Exhintake Cam. Go ahead and google it; yep, I created that.